Injection molding apparatus

Abstract: An injection moulding apparatus has a plasticizing space (3) formed in a substantial cone or disk shape between a heating cylinder (1) and a screw (2), into which moulding resin is introduced. The moulding resin in the plasticizing space (3) can be maintained in its uniformly plasticized state by continuously rotating the screw during injection moulding operation, thus increasing the efficiency of injection moulding. The heating cylinder (1) and screw (2) can be readily be assembled because the heating surface (11) of the heating cylinder and the plasticizing surface (21) of the screw are formed in a substantial cone or disk shape.

Description: This invention relates to an injection moulding apparatus having a unique plasticizing structure capable of constantly maintaining moulding resin in its uniformly plasticized state, thereby to produce moulded articles of high quality. There has been so far widely used an in-line screw type injection moulding apparatus having a rotating screw movable back and forth in a heating cylinder. Molding material supplied into a plasticizing space defined between the inner surface of the heating cylinder and the circumferential surface of the screw is plasticized with not only the heat essentially generated by the heating cylinder but also shearing calorific stress caused by the rotation of the screw. While feeding out the plasticized material toward the cylinder head, the rotating screw is forced backward by the reaction of the moulding material. Then, the screw stops rotating and thrust forward to inject the plasticized moulding material from the cylinder into a mould. In the conventional injection moulding apparatus of this type, the plasticizing space between the inner heating surface of the heating cylinder and the outer surface of the screw is formed in the shape of a thin tube extending parallel with the axis of the cylinder. In the conventional moulding apparatus, the tubular plasticizing space formed in the heating cylinder and extending parallel with the axis of the cylinder entails various problems as described below. The unit constituted by the heating cylinder and screw in the conventional moulding apparatus necessitates a rather long plasticizing passage for the moulding material in order to uniformly plasticize the moulding material and allow the screw to sufficiently move back and forth in the cylinder. Thus, the conventional moulding apparatus is inevitably complicated in structure and made large in size. In particular, as the screw having a feed zone, plasticization zone, and metering zone is necessarily made long, the heating cylinder and injection mechanism in the conventional apparatus become long. As one example, the injection mechanism capable of exerting a mould clamping force of 25 tons has generally a length of about 350 mm to 400 mm, and that capable of exerting a moulding clamping force of 200 tons becomes as long as about 1000 mm to 1500 mm in length. Furthermore, the plasticizing space between the heating cylinder and the screw must be narrowed down to less than 0.02 mm in general in order to allow the screw to move back and forth without degrading plasticizing efficiency. However, the work of inserting the screw into the heating cylinder having an inner diameter slightly larger than the outer diameter of the screw to assemble the moulding apparatus turns out to be very onerous. In addition, the metallic heating cylinder and metallic screw which are in close contact with each other is worn out during the course of prolonged service. Moreover, the screw is disadvantageous in that the plasticizing space is fixedly defined between the cylinder and thus, it cannot be adjusted in accordance with the quality or properties such as viscosity of the moulding resin. As a result, the moulding resin containing fiberglass or other specific composite resin is rather restricted in size of annexations such as fiberglass and other conditions. When such resin containing fiberglass is continuously dealt with, the screw is easily worn out as much as about 0.03 mm to 0.15 mm, consequently to degrade the plasticizing efficiency. In addition, since the plasticizing space formed in a tube extends parallel to the rotation axis of the screw, the screw has to stop rotating to cease the plasticization of the moulding material, with the result that the moulding material to be injected cannot be uniformly plasticized nor injected with a high efficiency. Besides, since the conventional moulding apparatus is generally provided with a check valve near a nozzle in the cylinder head, the injection of the molten resin cannot be stably performed due to different injecting conditions and disturbance. Therefore, in order to perform proper injection moulding for obtaining precise moulded article by utilization of the conventional moulding apparatus, injection moulding operation has had to be executed while keeping the moulding conditions under observation in such a manner that an excessive amount of moulding material is constantly supplied so that the moulding material can be added at all times in accordance with the cushioning action brought about by the moulding material being injected from the plasticizing space into the mould. Thus, supplementary moulding material is complementarily supplied according to the amount of the moulding material in the plasticizing space, as a result of which a moulded article is formed of different moulding resins in viscosity, thus causing unevenness on mouldings and irregularity in injection shots. In addition to the aforementioned in-line screw type moulding apparatus, there have been conventionally known a plunger type injection moulding apparatus using a plunger for injecting moulding material heated by torpedo, plunger preplasticating type injection moulding apparatus having a multi-cylinder structure formed by combining the plunger type and in-line screw type moulding apparatuses, and a screw preplasticating type injection moulding apparatus. However, these conventional moulding apparatuses have a common disadvantage of being inefficient in plasticizing the moulding material and complicated in structure, and therefore, cannot receive general application. US-A-3 355 764 discloses an injection moulding apparatus in accordance with the preamble of Claim 1. An object of the present invention is to provide an injection moulding apparatus having an improved plasticizing structure free from the disadvantages suffered by the conventional moulding apparatus as described above. Another object of this invention is to provide an injection moulding apparatus with shortened screw and heating cylinder, capable of being notably made simple in structure, readily assembled, miniaturized without degrading plasticizing efficiency, and easily adjusting a plasticizing space formed between the screw and heating cylinder in accordance with the quality or properties of the moulding material. Still another object of this invention is to provide an injection moulding apparatus capable of continuing uniform plasticization of the moulding material while injecting the moulding material. To attain the objects described above, the invention provides an injection moulding apparatus in accordance with Claim 1. The heating surface of the heating cylinder and the plasticizing surface of the screw may be formed aslant relative to the axis of the heating cylinder to define a substantial conical plasticizing space between the heating cylinder and the screw. The heating surface of the heating cylinder and the plasticizing surface of the screw may be arranged perpendicular to the axis of the heating cylinder so as to form a disk-shaped plasticizing space between the heating surfaces of the heating cylinder and the plasticizing surface of the screw. The heating cylinder may have an injection nozzle at the apex of the cone-shaped heating surface, through which moulding resin is injected into a mould cavity. The screw may be movable back and forth in the direction parallel to the axis of the heating cylinder, along which said plunger and said sleeve are movable. The moulding material is supplied into the plasticizing space formed in a substantial cone between the heating cylinder and screw and plasticized with shearing calorific stress produced by forcibly rotating the screw with large rotational torque. The moulding material thus plasticized is fed toward the injection nozzle disposed at the apex of the cone-shaped heating surface of the cylinder and gradually compressed in the substantially conical plasticizing space while increasing the density of the moulding material. Then, the moulding material fed into the nozzle is injected by the rotation of the screw and the forward movement of the plunger. When injecting the moulding material, the sleeve in the screw is moved forward to cut off the plasticizing space, thus preventing the moulding resin from flowing backward. One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment, in which:- FIG. 1 is a cross section showing the principal portion of a first embodiment of the injection moulding apparatus according to this invention; FIG. 2 shows the state in which the sleeve is protruded from the screw in the apparatus of FIG. 1; FIG. 3 shows the state in which the sleeve and plunger are protruded from the screw in the apparatus of FIG. 1; FIG. 4 is a top view showing the apparatus of FIG. 1; FIG. 5 is a front view showing a second embodiment of the apparatus according to this invention; FIG. 6 is a cross section showing the principal portion of FIG. 5; and FIG. 7 is a cross section showing the principal portion of a third embodiment of the apparatus according to this invention. The first embodiment of the present invention will be described hereinafter with reference to FIG. 1 through FIG. 4. Although this embodiment adopts a horizontal type injection moulding apparatus 10, this invention may of course be applied to a vertical type injection moulding apparatus as described later. The injection moulding apparatus 10 of this embodiment comprises a stationary platen 20, a movable platen 30, a mould unit 40 disposed between the stationary platen 20 and the movable platen 30, an injection mechanism 50 attached to the stationary platen 20, and a mould clamping mechanism 60 attached to the movable platen 30 as shown in FIG. 4. The mould unit 40 provided on its movable platen side with an ejection mechanism 70. The injection mechanism 50 is provided with an injection unit 80. As shown in FIG. 1 to FIG. 3 in detail, the injection mechanism 50 includes a heating cylinder 1, a screw 2, a plasticizing space 3 defined between the heating cylinder 1 and the screw 2, a nozzle 4, a hopper 5, a plunger 6, and a sleeve 7. The heating cylinder 1 has a heating surface 11 nonparallel to the centre axis thereof. In this embodiment, the heating surface 11 is shaped in a substantial cone having an apex or pointed end leading to the nozzle 4 and a gradually decreasing inner diameter toward the apex. The inclination of the heating surface 11 is determined to about 30 to 150°, preferably 60° to 120°. The heating cylinder 1 is provided.on its outer peripheral surface with a heater 13 held by a heater retainer 12. The screw 2 has a plasticizing surface 21 nonparallel to the rotation axis thereof which generally coincides with the centre axis of the heating cylinder 1. In this embodiment, the plasticizing surface 21 is shaped in a substantial cone gradually decreased in diameter toward its pointed end or apex. The inclination of the plasticizing surface 21 is substantially equal to that of the heating surface 11 of the cylinder 1 so that the screw 2 is fitted into the cylinder 1 with the heating cylinder 1 with the plasticizing surface 21 confronting the heating surface 11. The screw 2 is provided in the plasticizing surface 21 thereof with at least one spiral groove 22. The screw 2 is driven to not merely rotate about its axis but also move back and forth along the axis within the range defined by a gap 24 around a bearing 23 by driving means such as an electric motor (not shown). The screw 2 further includes heating means 25 such as a ceramic heater. In the united heating cylinder 1 and screw 2, the plasticizing space 3 is formed between the substantially cone-shaped heating surface 11 of the cylinder 1 and the substantially cone-shaped plasticizing surface 21 (spiral groove 22) of the screw 2. That is, the shape of the plasticizing space 3 assumes a substantial cone having a gradually decreasing diameter toward its apex. The conical plasticizing space 3 is connected at its apex to the nozzle 4 disposed on the leading end portion of the heating cylinder 1, and at its rear end to the hopper 5 mounted on the cylinder 1. The nozzle 4 includes a heater 42 and a thermocouple element 43, which are wound around the outer peripheral surface of a nozzle body 41. The plunger 6 having a substantially conical leading end portion which constitutes the leading end of the screw 2 is held movably in the axial direction of the screw and driven in the screw to move back and forth by means of the aforementioned injection unit 80. The sleeve 7 disposed around the plunger 6 is also held movably in the axial direction of the screw and driven by the injection unit 80 independent of the plunger 6 so as to cut off or ensure the plasticizing space 3. Since the heating cylinder 1 and the screw 2 are united along the conical surface like a wedge, the cylinder 1 and screw 2 can be assembled with very ease in comparison with a conventional in-line screw type injection moulding apparatus. Furthermore, the plasticizing space 3 defined between the cylinder 1 and the screw 2 need not be made thin precisely because of the plasticizing ability brought about by sufficient rotational torque which will be explained later. The plasticizing space 3 in this embodiment is determined to about 0.2 mm to 0.6 mm so as not to bring the cylinder 1 and screw 2 into metallic contact with each other. By moving the screw 2 back and forth within the gap 24, the plasticizing space 3 can be suitably adjusted with ease in accordance with the properties of moulding resin 100 such as viscosity. Moreover, since the plasticizing space 3 between the united cylinder 1 and screw 2 is formed aslant relative to the axis of the cylinder 1, the cylinder 1 and screw 2 can be remarkably reduced in length to 1/33 to 1/5 of those in the conventional injection moulding apparatus. Next, the operation of the apparatus in this embodiment will be described. First, the moulding resin 100 is put in the hopper 5 and introduced into the plasticizing space 3 by the rotation of the screw 2 while being heated by the heater 13 of the cylinder 1 and the heater 25 in the screw 2. The moulding resin 100 is stirred forcibly by the rotating screw 2, thus acquiring larger shearing calorific stress in the rear-side portion of the plasticizing space 3 at which the circumferential speed of the screw 2 is higher than that at the front-side portion. As a result, the moulding resin 100 is molten with the shearing calorific stress and plasticized uniformly with the aid of the heat generated by the heaters 13 and 25. The moulding resin 100 is forwarded toward the nozzle 4 along the plasticizing space 3 while being kept in its uniformly plasticized state. As the moulding resin 100 advances toward the apex of the substantially cone-shaped plasticizing space 3, the rotational torque produced by the rotating screw 2 is gradually decreased, but the density of the moulding resin 100 is gradually increased because the moulding resin forcibly forwarded is converged toward the apex of the conical plasticizing space 3. Thus, the moulding resin 200 sent into the nozzle 4 is maintained in its uniformly plasticized state. After the prescribed amount of plasticized moulding resin is fed out through the nozzle 4, the sleeve 4 is pushed forward to cut off the plasticizing space 3 as illustrated in FIG. 2, and then, the plunger 6 is protruded to inject the moulding resin 200 into the mould unit 40 as shown in FIG. 3. Thus, by arbitrarily determining the stroke within which the plunger 6 is protruded, the amount of moulding resin can be adjusted precisely. It is also possible to reserve the prescribed amount of molten resin between the leading end of the plunger 6 and the nozzle 4 by positively moving the plunger 6 backward while feeding out the moulding resin 200. Although the plunger 6 is pushed forward to inject the moulding resin 200 into the mould after protruding the sleeve 7, the moulding resin 200 is prevented from flowing backward by the protruded sleeve 7 to cut off the plasticizing space 3. While injecting, the moulding resin 200 is remained in its plasticized state, because the screw 2 does not have to stop rotating. Therefore, the injection of the moulding resin 200 into the mould can be effectively performed without a break even immediately after carrying out a preceding injection shot. In a conventional in-line screw type injection moulding apparatus, some moulding resin should be left in an injection nozzle or near a check valve to fulfill a cushioning function each time an injection shot is performed. However, such a conventional method entails a disadvantage such that the cushioning effect of the moulding resin left in the nozzle varies widely. According to the aforesaid embodiment of this invention, however, only the required quantity of moulding resin for one injection shot may be sent out into the nozzle 4. It was confirmed experimentally that, even when 1 cc of moulding resin is left in the nozzle 4, such a small quantity of moulding resin is completely dissipated in the nozzle 4 and in no way influencing an article resultantly produced. Therefore, the injection moulding apparatus of this invention is suitable for moulding by use of crystalline resin, which are easily pyrolytically decomposed, such as polybutylene terephthalate (PBT) and polyacetal (POM). For instance, the present moulding apparatus having a screw of 60 mm in length and 110 mm in diameter with a spiral groove 22 of 20 mm in width and 3 mm to 4 mm in depth and a plasticizing space determined to an incline of 34/100 can display compressibility nearly three-times as large as that in the conventional in-line screw type moulding apparatus. FIG. 5 and FIG. 6 show the second embodiment of the invention applied to a vertical type injection moulding apparatus. As shown in FIG. 5, the injection moulding apparatus of this embodiment is substantially identical with the foregoing embodiment except an injection mechanism arranged vertically. In this apparatus, a hydraulic unit including a pump and piping is disposed in a casing 400 having caster wheels 300. Since the injection mechanism is mounted on the movable casing 400, the entire moulding system can be made simple and easy to handle and transport. To be more specific, this injection moulding apparatus comprises a mould unit 40, and an injection mechanism 50 disposed on the mould unit and provided with a heating cylinder 1 having a vertically arranged axis. Since a plasticizing space 3 formed between a screw 2 and the heating cylinder 1 assumes the appearance of a conical funnel having a gradually decreasing diameter toward the apex located at the lower end portion, moulding resin which is plasticized by heating means is prevented from staying in the plasticizing space 3, and efficiently forced toward the nozzle 4 by the force of gravity. Therefore, the moulding resin can be more uniformly plasticized and effectively injected into the mould unit 40 in comparison with the horizontal type injection moulding apparatus in the foregoing embodiment. In this embodiment, the sleeve 7 has an annular step 7' for reliably cutting off the plasticizing space 3 as shown in FIG. 6. There is formed a spiral groove 22 in not only a conical plasticizing surface portion 21 at the leading end of the screw 2 but also a part of a cylindrical surface 26 parallel to the screw axis so as to connect the plasticizing space 3 with a resin supplying path 8 extending aslant upwards. Thus, the moulding resin is smoothly introduced into the plasticizing space 3 and prevented from flowing backward. A plunger 6 movable vertically is driven by vertically rocking a lever arm 60 engaged with stoppers 61 fixed on the plunger 6. The result of the experiment which was made for evaluating the present invention in comparison with the conventional injection moulding apparatus of this type will be described hereinbelow. In the experiment, there was used the injection moulding apparatus of this invention exhibiting a mould clamping force of 25 tons, injection capacity of 20 cc, heating temperature of 220°C, measuring time of 1.0 to 1.5 sec., and filling time of 0.5 sec. and having a screw 2 of 60 mm in length, 110 mm in diameter and 200 rpm in rotational speed. The spiral groove 22 of 20 mm in width and 3 mm to 4 mm in depth is formed continuously over the entire length of the screw 2 so as to form a conical plasticizing space 3 having an incline of 34/100. As a comparative apparatus, there was used an injection moulding machine, model "PROMAT 25t" made by Sumitomo. Heavy Industries, Ltd. of Japan, showing a mould clamping force of 25 tons, injection capacity of 20 cc, heating temperature of 220°C, measuring time of 2.2 to 2.3 sec., and filling time of 0.5 sec. and having a screw of 320 mm to 330 mm in length, 20 mm in diameter and 150 rpm in rotational speed. The screw in this conventional apparatus has a spiral groove of 1.5 mm to 4 mm in depth. The depth and shape of the spiral groove vary in each of feed zone, plasticization zone and metering zone. As a moulding material, polypropylene (PP) made by Tokuyama Soda Ace Polymer Co., Ltd. of Japan was used in the experiment, in which a master batch ratio was determined to 100:1. Specimens were obtained respectively by slicing mouldings resultantly produced to 0.05 mm thickness and examined in color distribution with a microscope magnifying fifty times. It was evident from the results of the experiment that the injection moulding apparatus of the present invention can produce a uniformly moulded article of good quality in which coloring matter is moderately distributed throughout. On the contrary, in an moulded article produced by the conventional injection moulding apparatus, gross uneven stripes of coloring and unfavorable moulded portions were found. It can be assumed that the molten resin 200 in the conventional apparatus is injected into the mould without changing its section in the screw groove, but that in the apparatus of this invention is compressed and injected while being twisted in the substantially conical plasticizing space. It was also confirmed experimentally that the molten resin 200 in the conventional apparatus stays behind in the heating cylinder in one injection shot because the check valve disposed in contact with the heating surface of the heating cylinder is dull of responding. The amount of molten resin stayed behind in the cylinder in the conventional apparatus nearly three times as large as that in the present invention. What should be noted is the fact that the entire injection moulding apparatus of this invention could be made notably compact into dimensions of about 1600 mm in height, about 700 mm in frontage and about 600 mm in depth. FIG. 7 shows the third embodiment in which a plasticizing space 3 is formed in a circle flat shape like a disk. That is to say, the plasticizing space 3 in this embodiment spreads radially perpendicular to the axis of a heating cylinder 1. This disk-shaped plasticizing space 3 has the same function and effect as those of the foregoing embodiments. The plasticizing space 3 is formed between a flat plasticizing surface 21 on the front end of a screw 2 and a flat heating surface 11 of the heating cylinder 1. In the plasticizing surface 21 of the screw 2, there is formed a spiral groove 22 having an outer peripheral end serving as a resin feeding port. The moulding resin is introduced into the plasticizing space 3 and forced toward the centre portion leading to a nozzle 4 along the spiral groove 22 while being compressed. In this embodiment, by rotating the screw 2, the moulding resin can be fed into the nozzle 4 without a plunger 6 and sleeve 7 as adopted in the foregoing embodiments. Since the whole plasticizing surface 21 of the screw 2 which is perpendicular to the axis of the screw 2 imparts a pushing force to the moulding resin 200 in the plasticizing space 3, the injection pressure is increased conveniently. In addition, since the molten resin 200 is guided by the spiral groove 22 and the rotating axis 2 produces large stress on the outer peripheral portion thereof, the moulding resin flowing into the plasticizing space 3 is no longer fed backward. That is, there is no necessity for providing a check valve or the like for the purpose of preventing the moulding resin from flowing backward. As is apparent from the foregoing description, the injection moulding apparatus according to this invention having a unique plasticizing structure in which the plasticizing space defined between the heating cylinder and the screw is aslant or perpendicular to the axis of the cylinder can effectively eliminate various disadvantages suffered by a tube-like plasticizing space appearing in the conventional in-line screw injection moulding apparatus. To be more specific, according to this invention, the heating cylinder and screw can be considerably shortened with increasing the plasticizing efficiency. Furthermore, the heating cylinder and screw can readily be assembled only by joining the plasticizing surface on the front end of the screw to the heating surface of the heating cylinder. According to this plasticizing structure, the plasticizing space can easily be adjusted in accordance with the quality or properties of the moulding material, and uniform plasticization of the molten resin can be maintained constantly during the injection moulding operation, thus greatly increasing the efficiency of continuous injection. Since the injection moulding apparatus according to this invention has no need for a check valve, the moulding resin staying behind in the plasticizing space can be decreased in quantity. Besides, the moulding resin to be injected into the mould can be precisely measured by operating the plunger and sleeve. By arranging the plasticizing structure according to this invention. vertically, since the molten resin is forced downward by the force of gravity, the efficiency of plasticizing and injecting the molten resin can be remarkably increased.

Claims: An injection moulding apparatus comprising a heating cylinder (1) having a centre axis and an inner heating surface (11) nonparallel to said centre axis, and a screw (2) having a rotation axis and a plasticizing surface (21) nonparallel to said rotation axis, wherein said screw (2) is provided with a plunger (6) movable in the direction of said rotation axis of said screw, and said screw (2) is fitted into said heating cylinder (1) with said plasticising surface confronting said heating surface (11) of said cylinder to define a plasticizing space (3) between said heating surface (11) of said heating cylinder and said plasticizing surface (21) of said screw into which moulding resin is introduced, characterized in that said screw (2) includes a sleeve (7) movable in the direction of said rotation axis of said screw so as to cut off said plasticizing space (3) by moving forward said sleeve. An apparatus according to Claim 1, wherein said heating surface (11) of said heating cylinder (1) and said plasticizing surface (21) are aslant relative to said centre axis and said rotation axis to have said plasticizing space (3) shaped in a substantial cone shape. An apparatus according to Claim 1, wherein said heating surface (11) of said heating cylinder (1) and said plasticizing surface (21) are formed perpendicular to said centre axis or said rotation axis to have said plasticizing space (3) shaped in a substantial disk shape.

Inventors: TAKEUCHI HIROSHI, TAKEUCHI, HIROSHI
Applicants: SHINKO SELLBIC CO LTD, SHINKO SELLBIC CO., LTD.
Application year: 1994
Main classification: B29C45
IPC codes: B29C45